Efforts have been made for a long time to harness the energy generated by ocean waves. Among the methods used traditionally for this purpose are on-shore systems.
However, on-shore systems and methods for converting wave energy suffer from certain drawbacks, foremost among which are the poor efficiency of the energy potential, their great environmental impact and the social rejection of said environmental impact.
To overcome these drawbacks, off-shore methods and systems have been proposed for harnessing wave energy. Off-shore systems can be of two types, submerged or based on floating structures. Submerged off-shore systems have the drawback that the off-shore energy distribution decreases exponentially with the distance to the surface. For this reason, off-shore systems based on floating structures are predominant, as they can capture the energy where its concentration is greatest.
Off-shore systems based on floating structures comprise a floating structure, subjected to the effect of waves, on which one or several wave energy converters are mounted. Three main types of floating off-shore systems can be identified: (a) point absorbers, (b) attenuators and (c) terminators. FIG. 1 shows a diagram of the wave direction (direction of the arrow) with respect to the wave front for each type of floating off-shore system. The attenuators (represented in FIG. 1(b)) are elongated structures placed parallel to the direction of travel of the waves, extracting energy in a gradual manner, so that their equivalent capture width is considerably greater than that of the point absorbers (represented in FIG. 1(a)). According to David Evans in “The hydrodynamic efficiency of wave-energy devices”, Hydrodynamics of Ocean Wave-Energy Utilization, Lisbon, Springer-Verlag: 1-34, 1985, ideally their absorption capacity can be up to three times more than point absorbers. In short, this means a greater power generation capacity per unit weight of the converter. On the other hand, these systems are less exposed to damage and require lower mooring efforts than terminators.
However, most floating off-shore systems require a fixed reference for their operation (anchoring to sea base or ballast), which increases the mooring loads. In addition, these devices are sensitive to tides and their installation and maintenance are more complex.
There are some off-shore floating systems that do not need an external reference, based on either the relative motion between two or more bodies or on inertial motion.
Inertial systems have fully encapsulated mobile components, increasing protection against marine corrosion, thereby reducing maintenance costs, risk of malfunction and risk of polluting the environment (for example fluid leaks). Known inertial systems are based exclusively on the oscillating mass principle (sliding on a guide or pendulum). As gravity is the restoring force, their yield is low unless the oscillating mass is acted upon at zero-velocity points (by latching). However, their control is very complex as it is hard to find the optimum point at which to release the mass, given the variability of the waves.
On the other hand, also known are flywheels for stabilising the rocking motion of ships. These flywheels, known as gyroscopes, use the restoring force resulting from the gyroscopic effect which ideally would allow a full transmission of the wave's external torque to another internal component. In addition, it is relatively easy to keep them in phase by an active control of the rotational speed and the resistance torque. U.S. Pat. No. 4,352,023 describes a floating body joined to a power transducer that allows transforming the motion of the waves into the rotation of a shaft. This system is based on two casings, one of which is joined to a gyroscope coupled to a motor that allows it to rotate. Due to the gyroscopic effect, as the flywheel is constantly turning, the pitch and roll motion to which the floating body is subjected by the waves becomes an oscillating motion along an outlet shaft perpendicular to the drive motor shaft. By an appropriate coupling, the oscillating motion is transformed into a unidirectional rotation that drives an electrical generator.
However, this system has several drawbacks, as the gyroscope of said system cannot adapt to external conditions such as wave type, frequency or height. For this reason, the use of wave energy is very low. To adapt to external conditions, this is, to the excitatory moment of the wave, it is necessary to adapt two factors: the rotation speed and the resistance torque. Controlling the rotation speed allows controlling the amplitude of the oscillation or rolling to obtain a maximum energy capture. Controlling the resistance torque allows maintaining the oscillation of the converter in phase with the excitatory moment to obtain a positive balance of captured energy.
However, the system described in U.S. Pat. No. 4,352,023 does not consider controlling the capturing device according to the external wave conditions, so that its energy efficiency is very low. In addition, its control system acts only on the rotation speed, without controlling the resistance torque, which is a very important factor. For this reason, the efficiency of U.S. Pat. No. 4,352,023 for harnessing wave energy is very low.